The present invention relates to an optical pickup apparatus and a resin-made objective lens, and in particular, to an objective lens improved in terms of temperature characteristics and aberrations and to an optical pickup apparatus which employs the objective lens mentioned above and is used favorably for recording and/or reproducing an optical information recording medium.
As an optical system for recording and reproducing an optical information recording medium having accuracy required for a conventional CD reproducing apparatus (incidentally, the optical system for recording and reproducing mentioned in the present specification includes an optical system for recording, an optical system for reproducing, an optical system for recording and reproducing, or an apparatus employing them), an optical system of an infinite conjugation type is disclosed in TOKKAISHO No. 57-76512, and an optical system of a finite conjugation type is disclosed in TOKKAISHO No. 61-56314. Further, TOKKAIHEI No. 6-258573 discloses one employing a coupling lens for reducing occurrence of aberrations which are caused by temperature changes when a resin-made objective lens is used. In recent years, however, a lens made of resin (plastic) material is commonly used for the requirement of low cost with regard to an optical system for recording and reproducing, especially to an objective lens for the optical system for recording and reproducing.
However, an objective lens made of resin materials has a problem that an aberration generated by refractive index changes caused by temperature changes is greater than that for a lens made of glass material. In general, a refractive index change for resin materials is ten times or more that for glass materials. When xcex94T represents a difference of temperature between standard design temperature and actual ambient temperature used, an aberration is changed by this temperature difference xcex94T is mainly third order spherical aberration. When SA represents third order spherical aberration components of wavefront aberration expressed in an rms value, a sign is defined so that SA is greater than zero when spherical aberration is positive (over) and SA is smaller than zero when spherical aberration is negative (under). It is possible to express third order spherical aberration xcex94SA rms that changes depending on temperature change xcex94T with the following expression, by using numerical aperture NA of an objective lens on the optical information recording medium side (image side), focal length f, image forming magnification m, factor of proportionality k and wavelength of light xcex.
xcex94SA/xcex94T=kxc2x7f(1xe2x88x92m)4(NA)4/xcexxe2x80x83xe2x80x83(1)
Incidentally, when a lens made of resin materials has positive refracting power, the third order spherical aberration turns out to be over when a temperature rises. Namely, in the above expression (1), factor of proportionality k takes a positive value. When a single lens made of resin materials is made to be an objective lens, the factor of proportionality k takes a large positive value.
In the objective lens for a compact disk that is presently used commonly, aberrations generated by changes in ambient temperatures used do not go up to the problematic level, because NA is about 0.45. However, at the present time wherein optical information recording media are in the trend toward high density, an objective lens constituting an optical system of an apparatus for recording and reproducing is also requested to comply with the trend of high density.
To be concrete, there has been developed, as an optical information recording medium, a DVD (storage capacity: 640 MB) which is similar to CD (storage capacity: 4.7 GB) in terms of size and has been enhanced in terms of recording density, and the DVD is spreading rapidly. For reproducing the DVD, it is general to use a laser beam with prescribed wavelength emitted from a light source whose wavelength is within a range from 635 nm to 660 nm. Further, it is general that a divergent light flux emitted from a laser light source is collimated by a collimator lens, then, is made to enter an objective lens whose NA on the DVD side is 0.6 or more, and is converged on an information recording surface through a transparent base board of DVD.
In recent years, in particular, an optical information recording medium which is the same as CD and DVD having a storage capacity of 10-30 GB is being developed actively by using an objective lens having higher NA and a light source having a shorter wavelength. As a light source having a short wavelength which is considered to be promising, there are available GaN blue semiconductor laser-with oscillation wavelength of about 400 nm and SHG blue laser. Namely, an optical system in an apparatus for recording and reproducing is requested to have high NA and requested to comply with a laser beam whose wavelength is further short.
In consideration of the foregoing from the viewpoint of wavefront aberration, when NA is increased from 0.45 to 0.6 and wavelength xcex of the laser beam is shortened from 660 nm to 400 nm, for example, wavefront aberration Wrms is increased by (0.6/0.45)4÷400/660=5.17 times.
Though it is considered that focal length f is made small for controlling wavefront aberration to be small based on expression (1), it is actually difficult to make f to be smaller than the current size, because of necessity to secure a focusing operation distance. Further, in the optical system of a finite conjugation type of m less than 0 and in the optical system of a infinite conjugation type of m=0, aberrations caused by temperature changes are serious problems in the case of high NA. Though it is considered that temperature characteristics are improved by realizing 0 less than m less than 1 in the optical system employing a coupling lens, there is a problem, in this case, that the optical system, and further, the apparatus need to be large in size, because a distance between a subject and an image of an optical system needs to be increased in length, or a coupling lens with high NA is needed, for securing an operation distance necessary for focusing.
As stated above, it has been difficult to realize an optical system having high NA in the lens system wherein a conventional resin-made objective lens is used, because of occurrence of an aberration that is in proportion to 4th power of numerical aperture NA of an objective lens on the image side caused by refractive index change xcex94n of resin material that is further caused by temperature change.
In the optical system of an optical information recording and reproducing apparatus that is requested to attain high density information recording by the short wavelength of a laser light source and by high NA, it is unavoidable to give up the use of resin-made objective lens and to use a glass-mold lens or a glass combined lens wherein a refractive index change for temperature change is small but the cost is high.
For the problems mentioned above, TOKKAIHEI No. 11-337818, for example, discloses a technology for correcting aberrations for temperature change by providing a diffraction lens structure having a prescribed spherical aberration characteristics on an objective lens for an optical head.
However, there also is a demand to reduce the number of lenses in a light-converging optical system for the purpose of achieving compactness and low cost of a recording and reproducing apparatus for an optical information recording medium. To satisfy this demand, it is necessary to constitute an optical system of the so-called finite conjugation type by the use of a single objective lens. However, the objective lens disclosed in TOKKAIHEI No. 11-337818 does not comply with the optical system of a finite conjugation type.
In an objective lens for a recording and reproducing apparatus (optical pickup apparatus) for an optical information recording medium, an object of the invention is to provide a resin-made objective lens that can secure sufficient capacity for a change of ambient temperature used even when a light source of the apparatus is an SHG laser whose oscillation wavelength is not changed by a change of ambient temperature used, or a semiconductor laser whose oscillation wavelength is changed, or to provide an objective lens including at least one lens made of resin material, and an optical pickup apparatus employing the objective lens mentioned above. Namely, the object is to provide an objective lens, an optical pickup apparatus and a recording and reproducing apparatus for an optical information recording medium which can lessen a change in third order spherical aberration and further a change in spherical aberration for a change of ambient temperature used, independently of the types of a laser light source.
An object of the invention is to provide an objective lens that is made of resin materials capable of securing sufficient capacity for a change of ambient temperature used and is able to constitute an optical system of a finite conjugation type, and to provide an optical pickup apparatus employing the objective lens stated above.
The above object can be attained by the following structures.
(1-1) An optical pickup apparatus to conduct reproducing and/or recording information of an optical information recording medium, comprises:
a light source to emit a light flux having a wavelength of xcex (nm);
a converging optical system to converge the light flux emitted from the light source onto a information recording surface of the optical information recording medium, the converging optical system having an objective lens; and
an optical detector to receive reflected light from the optical information recording medium; wherein the objective lens is a plastic lens, has a diffracting section on at least one surface thereof and satisfies the following conditional formula:
0.3xe2x89xa6xcfx86R/xcfx86xe2x89xa61.5
where xcfx86R represents a refracting power of the objective lens on wavelength xcex (nm) of the light source and xcfx86 represents a power of the objective lens in wavelength xcex nm of the light source.
(1-2) In the optical pickup apparatus of (1-1), the following conditional formula is satisfied:
(xcex94SA2/xcex94xcex1)xc3x97(xcex94SA1/xcex94T) less than 0 xcexrms2/(xc2x0 C.xc2x7nm)
where xcex94SA2 represents a fluctuation amount of third order axial spherical aberration of the objective lens for wavelength variation xcex94,xcex1 (nm) within a range of xc2x15 nm in wavelength xcex (nm) of a light source, and xcex94SA1 represents a fluctuation amount of third order axial spherical aberration of the objective lens for temperature variation xcex94T(xc2x0 C.) within a range of 20xc2x0 C. to 30xc2x0 C. for ambient temperature, wherein a value of (xcex94SA2/xcex94xcex1) is under a constant temperature and a value of (xcex94SA1/xcex94T) is under a constant wavelength.
(1-3) in the optical pickup apparatus of (1-2), a numerical aperture NA of the objective lens at the optical information recording medium-side is not smaller than 0.58 and an image forming magnification mo1 of the objective lens is almost 0 (zero).
(1-4) In the optical pickup apparatus of (1-2), the image forming magnification mo1 of the objective lens satisfies the following conditional formula:
xe2x88x92xc2xdxe2x89xa6mo1xe2x89xa6xe2x88x92{fraction (1/7.5)}
(1-5) In the optical pickup apparatus of (1-2), the following conditional formula is satisfied:
xcex94SA2/xcex94xcex1 less than 0 xcexrms/nm
xcex94SA1/xcex94T greater than 0 xcexrms/xc2x0 C.
(1-6) In the optical pickup apparatus of (1-1), the following conditional formula is satisfied:
|xcex94SA1/xcex94T|xe2x89xa60.001 xcexrms/xc2x0 C.
where xcex94SA1 represents a fluctuation amount of third order axial spherical aberration of the objective lens for temperature variation xcex94T(xc2x0 C.) within a range of 20xc2x0 C. to 30xc2x0 C. for ambient temperature, wherein a value of (xcex94SA1/xcex94T) is under a constant wavelength.
(1-7) In the optical pickup apparatus of (1-6), the following conditional formula is satisfied:
|xcex94SA1/xcex94T|xe2x89xa60.004 xcexrms/xc2x0 C.
(1-8) In the optical pickup apparatus of (1-6), a numerical aperture NA of the objective lens at the optical information recording medium-side is not smaller than 0.58 and an image forming magnification mo1 of the objective lens is almost 0 (zero).
(1-9) In the optical pickup apparatus of (1-6), the wavelength xcex (nm) of the light source is not larger than 680 (nm).
(1-10) In the optical pickup apparatus of (1-6), a numerical aperture NA of the objective lens at the optical information recording medium-side is not smaller than 0.48 and smaller than 0.58, an image forming magnification mo1 of the objective lens is almost 0 (zero), and the following conditional formula is satisfied:
|xcex94SA1/xcex94T|xe2x89xa60.0004 xcexrms/xc2x0 C.
(1-11) In the optical pickup apparatus of (1-6), a numerical aperture NA of the objective lens at the optical information recording medium-side is not smaller than 0.49, an image forming magnification mo1 of the objective lens satisfies the following conditional formula:
xe2x88x92xc2xdxe2x89xa6mo1xe2x89xa6xe2x88x92{fraction (1/7.5)}
and the following conditional formula is satisfied.
|xcex94SA1/xcex94T|xe2x89xa60.0005 xcexrms/xc2x0 C.
(1-12) In the optical pickup apparatus of (1-11), the objective lens is shiftable in a direction perpendicular to an optical axis thereof, a relative position between the objective lens and the light source is changeable and an astigmatism component of a wavefront aberration of a light flux which have been emitted from the light source and have passed through the objective lens becomes smallest at a position where the optical axis of the objective lens deviates from a center of the light flux of the light source.
(1-13) In the optical pickup apparatus of (1-11), the following conditional formula is satisfied:
10 mm less than U less than 40 mm
where U is a distance of an optical path between the light source and a information recording surface of the optical information recording medium.
(1-14) In the optical pickup apparatus of (1-11), the optical pickup apparatus is an optical pickup apparatus to conduct reproducing and/or recording information of a first information recording medium including a transparent base plate having a thickness of t1 and a second information recording medium including a transparent base plate having a thickness of t2 (t2 greater than t1) and further comprises a second light source to emit a second light flux having a wavelength of xcex2 (xcex less than xcex2) in addition to the light source to emit the light flux having the wavelength of xcex;
the converging optical system converges light flux emitted from the light source or the second light source onto an information recording surface of the first optical information recording medium or the second optical information recording medium;
the optical detector receives reflected light from the first optical information recording medium or the second optical information recording medium;
the NA is a numerical aperture of the objective lens at the first optical information recording medium-side and necessary to record or reproduce information in the first optical information recording medium with the light flux of the wavelength of xcex;
the NA2 (NA2 less than NA) is a numerical aperture of the objective lens at the second optical information recording medium-side and necessary to record or reproduce information in the second optical information recording medium with the light flux of the wavelength of xcex2;
the mo1 is the image forming magnification when recording or reproducing information in the first optical information recording medium; and
the following conditional formula is satisfied:
NAxe2x89xa70.56
xe2x80x83xe2x88x92⅕xe2x89xa6mo1xe2x89xa6xe2x88x92{fraction (1/7.5)}
(1-15) In the optical pickup apparatus of (1-14), wherein the following conditional formula is satisfied:
|mo2xe2x88x92mo1| less than 0.1
where mo2 is a image forming magnification when recording or reproducing the second optical information recording medium.
(1-16) In the optical pickup apparatus of (1-1), the following conditional formula is satisfied:
0 nm/xc2x0 C. less than xcex94xcex2/xcex94Txe2x89xa60.5 nm/xc2x0 C.
where xcex94xcex2 (nm) represents a wavelength variation amount of the light source for temperature variation xcex94T(xc2x0 C.) within a range of 20xc2x0 C. to 30xc2x0 C. of ambient temperature.
(1-17) In the optical pickup apparatus of (1-1), the following conditional formula is satisfied:
xe2x88x920.002/xc2x0 C.xe2x89xa6xcex94n/xcex94Txe2x89xa6xe2x88x920.00005/xc2x0 C.
where xcex94n represents a refractive index variation amount of the objective lens for temperature variation xcex94T(xc2x0 C.) within a range of 20xc2x0 C. to 30xc2x0 C. of ambient temperature.
(1-18) In the optical pickup apparatus of (1-17), the following conditional formula is satisfied:
xe2x88x920.0002/xc2x0 C. less than xcex94n/xcex94T less than xe2x88x920.00005/xc2x0 C.
(1-19) In the optical pickup apparatus of (1-1), the wavelength xcex of the light source and the numerical aperture NA of the objective lens at the optical information recording medium-side satisfy the following conditional formula:
0.00015/nmxe2x89xa6(NA)4/xcexxe2x89xa640/nm
(1-20) In the optical pickup apparatus of (1-1), the wavelength xcex of the light source and the numerical aperture NA of the objective lens at the optical information recording medium-side satisfy the following conditional formula:
10 nmxe2x89xa6xcex/NAxe2x89xa61100 nm
(1-21) In the optical pickup apparatus of (1-1), the following conditional formula is satisfied:
|xcex94SA2/xcex94xcex1|xe2x89xa60.10 xcexrms/nm
where xcex94SA2 represents a fluctuation amount of third order axial spherical aberration of the objective lens for wavelength variation Axcex1 nm within a range of xc2x15 nm in wavelength xcex nm of the light source, wherein xcex94SA2/xcex94xcex is a value under a constant temperature.
(1-22) In the optical pickup apparatus of (1-1), the following conditional formula is satisfied:
|xcex94f/xcex94xcex1xe2x89xa6200
where xcex94f (xcexcm) represents a focus position variation amount of the objective lens for wavelength variation xcex94xcex1 nm within a range of xc2x15 nm of wavelength xcex nm of the light source.
(1-23) In the optical pickup apparatus of (1-1), at least one surface of the objective lens is an aspherical surface.
(1-24) In the optical pickup apparatus of (1-1), the diffracting section is a plurality of ring-shaped bands in a form of concentric circles whose centers are mostly on an optical axis of the objective lens, and an optical path difference function showing a position of each of the plurality of ring-shaped bands includes at least 6th power term of power series.
(1-25) In the optical pickup apparatus of (1-1), the objective lens is a single lens.
(1-26) In the optical pickup apparatus of (1-1), the following conditional formula is satisfied:
1.0xe2x89xa6r1/{(nxe2x88x921)xc2x7fR}xe2x89xa61.2
where r1 (mm) represents an axial radius of curvature on the base surface of the objective lens at an opposite side to an optical information recording medium-side, n represents a refractive index of the objective lens and fR (mm) represents a focal length for refraction of the objective lens.
(1-27) In the optical pickup apparatus of (1-1), the objective lens is made of either one of polyolefin resin, norbornane resin and fluorine resin.
(1-28) In the optical pickup apparatus of (1-1), an axial spherical aberration change amount caused by ambient temperature variation is corrected by making at least one surface of the objective lens to be an aspherical surface and a spherical aberration is corrected by providing a diffracting section on at least one surface of the objective lens.
(1-29) In the optical pickup apparatus of (1-1), when a numerical aperture of the objective lens at the optical information recording medium-side necessary to conduct recording and reproducing the optical information recording medium with the light flux of the wavelength xcex is NA, and when a light flux which is emitted from the light source under ambient temperature of 20xc2x0 C. to 30xc2x0 C., is inside of NA and is converged on a information recording surface of the optical information recording medium, an absolute value of a third order axial spherical aberration is not larger than 0.074 xcexrms.
(1-30) In the optical pickup apparatus of (1-1), the optical pickup apparatus is an optical pickup apparatus to conduct reproducing and/or recording information of a first information recording medium including a transparent base plate having a thickness of t1 and a second information recording medium including a transparent base plate having a thickness of t2 (t2 greater than t1) and further comprises a second light source to emit a second light flux having a wavelength of xcex2 (xcex less than xcex2) in addition to the light source to emit the light flux having the wavelength of xcex;
the converging optical system converges light flux emitted from the light source or the second light source onto an information recording surface of the first optical information recording medium or the second optical information recording medium; and
the optical detector receives reflected light from the first optical information recording medium or the second optical information recording medium,
wherein the optical pickup apparatus conducts recording and/or reproducing information of the first optical information recording medium by using the light flux having the wavelength of xcex and conducts recording and/or reproducing information of the second optical information recording medium by using the light flux having the wavelength of xcex2.
(1-31) In the optical pickup apparatus of (1-30), a numerical aperture of the objective lens at the first optical information recording medium-side necessary to record or reproduce information in the first optical information recording medium with the light flux of the wavelength of xcex is NA and a numerical aperture of the objective lens at the second optical information recording medium-side necessary to record or reproduce information in the second optical information recording medium with the light flux of the wavelength of xcex2 is NA2 (NA2 less than NA), and
wherein when a light flux which is emitted from the light source under ambient temperature of 20xc2x0 C. to 30xc2x0 C., is inside of NA and is converged on a information recording surface of the first optical information recording medium, an absolute value of a third order axial spherical aberration is not larger than 0.07 xcexrms, and when a light flux which is emitted from the second light source under ambient temperature of 20xc2x0 C. to 30xc2x0 C., is inside of NA2 and is converged on a information recording surface of the second optical information recording medium, an absolute value of a third order axial spherical aberration is not larger than 0.07 xcexrms.
(1-32) An objective lens for use in an optical pickup apparatus to conduct reproducing and/or recording information of an optical information recording medium, comprises
lens surfaces; and
a diffracting section;
wherein the lens is a plastic lens, the diffracting section is provided on at least one of the lens surfaces and the following conditional formula is satisfied:
0.3xe2x89xa6xcfx86R/xcfx86xe2x89xa61.5
where xcfx86R represents a refracting power of the objective lens on wavelength xcex (nm) of the light source and xcfx86 represents a power of the objective lens in wavelength xcex nm of the light source.
(1-33) In the objective lens of (1-32), the following conditional formula is satisfied:
(xcex94SA2/xcex94xcex1)xc3x97(xcex94SA1/xcex94T)xe2x89xa60 xcexrms2/(xc2x0 C.xc2x7nm)
where xcex94SA2 represents a fluctuation amount of third order axial spherical aberration of the objective lens for wavelength variation xcex94xcex1 (nm) within a range of xc2x15 nm in wavelength xcex (nm) of a light source, and xcex94SA1 represents a fluctuation amount of third order axial spherical aberration of the objective lens for temperature variation xcex94T(xc2x0 C.) within a range of 20xc2x0 C. to 30xc2x0 C. for ambient temperature, wherein a value of (xcex94SA2/xcex94xcex1) is under a constant temperature and a value of (xcex94SA1/xcex94T) is under a constant wavelength.
(1-34) In the objective lens of (1-32), the following conditional formula is satisfied:
|xcex94SA1/xcex94T|xe2x89xa60.001 xcexrms/xc2x0 C.
where xcex94SA1 represents a fluctuation amount of third order axial spherical aberration of the objective lens for temperature variation xcex94T(xc2x0 C.) within a range of 20xc2x0 C.-30xc2x0 C. for ambient temperature, wherein a value of (xcex94SA1/xcex94T) is under a constant wavelength).
(1-35) A recording and/or reproducing apparatus to conduct reproducing and recording information of an optical information recording medium, comprises:
an optical pickup apparatus, comprising:
a light source to emit a light flux having a wavelength of xcex (nm);
a converging optical system to converge the light flux emitted from the light source onto a information recording surface of the optical information recording medium, the converging optical system having an objective lens; and
an optical detector to receive reflected light from the optical information recording medium; wherein the objective lens is a plastic lens, has a diffracting section on at least one surface thereof and satisfies the following conditional formula:
0.3xe2x89xa6xcfx86R/xcfx86xe2x89xa61.5
where xcfx86R represents a refracting power of the objective lens on wavelength xcex (nm) of the light source and xcfx86 represents a power of the objective lens in wavelength xcex nm of the light source.
Further, the above object can be also attained by the following preferable structures.
(2-1) An optical pickup apparatus has therein a light source that emits a light flux having wavelength xcex nm, an objective lens that converges the light flux emitted from the light source on an information recording surface of an optical information recording medium through a transparent base board of the optical information recording medium, and a photo-detector that receives a reflected light from the optical information recording medium, wherein the objective lens includes a plastic lens having refracting power, a diffraction pattern is provided on at least one surface of the objective lens, numerical aperture NA of the objective lens on the optical information recording medium side satisfies
NAxe2x89xa70.58
and the following expression is satisfied
(xcex94SA2/xcex94xcex1)xc3x97(xcex94SA1/xcex94T)xe2x89xa60 xcexrms2/(xc2x0 C.xc2x7nm)
when xcex94SA2 represents a fluctuation amount of third order axial spherical aberration of the objective lens for wavelength variation xcex94xcex1 nm within a range of xc2x15 nm for wavelength xcex nm of a light source, and xcex94SA1 represents a fluctuation amount of third order axial spherical aberration of the objective lens for temperature variation xcex94T(xc2x0 C.) within a range of 20xc2x0 C.-30xc2x0 C. for ambient temperature.
(2-2) In the optical pickup apparatus described in (2-1), the third order axial spherical aberration change amount xcex94SA2 satisfies the following,
xcex94SA2/xcex94xcex1 less than 0xcexrms/nm
and the third order axial spherical aberration change amount xcex94SA1 satisfies the following.
xcex94SA1/xcex94T greater than 0 xcexrms/xc2x0 C.
(2-3) In the optical pickup apparatus described in (2-1), when xcex94xcex2 nm represents a wavelength change amount of the light source for temperature change xcex94Txc2x0 C. within a range of 20xc2x0 C.-30xc2x0 C. of ambient temperature, the following expression is satisfied,
0 nm/xc2x0 C. less than xcex94xcex2/xcex94Txe2x89xa60.5 nm/xc2x0 C.
(2-4) In the optical pickup apparatus described in (2-1), the third order axial spherical aberration change amount xcex94SA1 satisfies the following expression.
|xcex94SA1/xcex94T|xe2x89xa60.001 xcexrms/xc2x0 C.
(2-5) An optical pickup apparatus having therein a light source that emits a light flux having wavelength xcex nm, an objective lens that converges the light flux emitted from the light source on an information recording surface of an optical information recording medium through a transparent base board of the optical information recording medium, and a photo-detector that receives a reflected light from the optical information recording medium, wherein the objective lens includes a plastic lens having refracting power, a diffraction pattern is provided on at least one surface of the objective lens, numerical aperture NA of the objective lens on the optical information recording medium side satisfies
NAxe2x89xa70.58
and the following expression is satisfied,
|xcex94SA1/xcex94T|xe2x89xa60.001 xcexrms/xc2x0 C.
where xcex94SA1 represents a fluctuation amount of third order axial spherical aberration of the objective lens for temperature variation xcex94T(xc2x0 C.) within a range of 20xc2x0 C.-30xc2x0 C. for ambient temperature.
(2-6) An optical pickup apparatus has therein a light source that emits a light flux having wavelength xcex nm, an objective lens that converges the light flux emitted from the light source on an information recording surface of an optical information recording medium through a transparent base board of the optical information recording medium, and a photo-detector that receives a reflected light from the optical information recording medium, wherein the objective lens includes a plastic lens having refracting power, a diffraction pattern is provided on at least one surface of the objective lens, numerical aperture NA of the objective lens on the optical information recording medium side satisfies
0.58 greater than NAxe2x89xa70.48
and the following expression is satisfied,
|xcex94SA1/xcex94T|xe2x89xa60.00040 xcexrms/xc2x0 C.
where xcex94SA1 represents a fluctuation amount of third order axial spherical aberration of the objective lens for temperature variation xcex94T(xc2x0 C.) within a range of 20xc2x0 C.-30xc2x0 C. for ambient temperature.
(2-7) An optical pickup apparatus has therein a light source that emits a light flux having wavelength xcex nm, an objective lens that converges the light flux emitted from the light source on an information recording surface of an optical information recording medium through a transparent base board of the optical information recording medium, and a photo-detector that receives a reflected light from the optical information recording medium, wherein the objective lens includes a plastic lens having refracting power, a diffraction pattern is provided on at least one surface of the objective lens, wavelength xcex nm of the above-mentioned light source satisfies the following expression,
xcexxe2x89xa6680 nm
and the following expression is satisfied,
|xcex94SA1/xcex94T|xe2x89xa60.001 xcexrms/xc2x0 C.
where xcex94SA1 represents a fluctuation amount of third order axial spherical aberration of the objective lens for temperature variation xcex94Txc2x0 C. within a range of 20xc2x0 C.-30xc2x0 C. for ambient temperature.
(2-8) In the optical pickup apparatus described in (2-7), wavelength xcex nm of the light source stated above satisfies the following expression showing the shorter wavelength.
xcexxe2x89xa6500 nm
(2-9) In the optical pickup apparatus described in one of (2-1) to (2-8), wavelength xcex nm of the light source and numerical aperture NA of the objective lens on the optical information recording medium side satisfies the following expression,
xe2x80x830.00015/nmxe2x89xa6(NA)4/xcexxe2x89xa640/nm
(2-10) In the optical pickup apparatus described in one of (2-1) to (2-9), wavelength xcex nm of the light source and numerical aperture NA of the objective lens on the optical information recording medium side satisfies the following expression,
10 nmxe2x89xa6NAxe2x89xa61100 nm
(2-11) In the optical pickup apparatus described in one of (2-1) to (2-10), the following expression is satisfied when xcex94SA2 represents a third order axial spherical aberration change amount of the objective lens for wavelength change xcex94xcex nm within a range of xc2x15 nm of wavelength xcex nm of the light source,
|xcex94SA2/xcex94xcex|xe2x89xa60.10 xcexrms/nm
(2-12) In the optical pickup apparatus described in one of (2-1) to (2-11), the following expression is satisfied when xcex94f xcexcm represents a focus position change amount of the objective lens for wavelength change xcex94xcex1 nm within a range of xc2x15 nm of wavelength xcex nm of the light source,
|xcex94f/xcex94xcex1xe2x89xa6200
(2-13) In the optical pickup apparatus described in one of (2-1) to (2-12), at least one surface of the objective lens is an aspherical surface.
(2-14) In the optical pickup apparatus described in one of (2-1) to (2-12), at least two surfaces of the objective lens represent an aspherical surface.
(2-15) In the optical pickup apparatus described in one of (2-1) to (2-12), the diffraction pattern is formed to be a plurality of ring-shaped bands in a shape of concentric circles whose centers are mostly on an optical axis, and the optical path difference function showing the position of each ring-shaped band of the plural ring-shaped bands includes at least 6th power term of the power series.
(2-16) In the optical pickup apparatus described in one of (2-1) to (2-15), the objective lens is composed of a single lens representing the plastic lens.
(2-17) In the optical pickup apparatus described in (2-16), the following expression is satisfied when xcfx86 represents a power of the objective lens in wavelength xcex nm of the light source and xcfx86R represents the refracting power,
0.3xe2x89xa6xcfx86R/xe2x89xa6xcfx86xe2x89xa61.5
(2-18) In the optical pickup apparatus described in (2-16) or (2-17), the following expression is satisfied when r1 represents an axial radius of curvature on the base surface of the objective lens opposite to an optical information recording medium, n represents the refractive index of the objective lens and fR mm represents a focal length for refraction,
1.0xe2x89xa6r1/{(nxe2x88x921)xc2x7fR}xe2x89xa61.2
(2-19) In the optical pickup apparatus described in one of (2-16) to (2-18), the following expression is satisfied when xcex94n represents an amount of refractive index change of the objective lens for temperature change xcex94T(xc2x0 C.) within a range of 20xc2x0 C.-30xc2x0 C. of ambient temperatures,
xe2x88x920.002/xc2x0 C.xe2x89xa6xcex94n/xcex94Txe2x89xa6xe2x88x920.00005/xc2x0 C.
(2-20) In the optical pickup apparatus described in one of (2-16) to (2-19), the objective lens stated above is made of either one of amorphous polyolefin resin, norbornane resin and fluorine resin.
(2-21) In the optical pickup apparatus described in one of (2-1) to (2-20), a flange section is provided on an outer circumference of an optical functional section of the objective lens.
(2-22) In the optical pickup apparatus described in (2-16), a cutout section is provided on a part of the flange section.
(2-23) An objective lens for use in an optical pickup apparatus comprises a plastic single lens having a diffracting pattern on at least one surface thereof and the following expression is satisfied when the objective lens is composed of a plastic single lens having refracting power and having a diffraction pattern on at least one surface, and r1 mm represents an axial radius of curvature of the base surface on the side opposite to that where an optical information recording medium is arranged, n represents refractive index and fR mm represents a focal length for refraction,
1.0xe2x89xa6r1/{(nxe2x88x921)xc2x7fR}xe2x89xa61.2
(2-24) In the optical pickup apparatus described in (2-23), the diffraction pattern is formed to be a plurality of ring-shaped bands in a shape of concentric circles whose centers are on an optical axis, and the optical path difference function showing the position of each ring-shaped band of the plural ring-shaped bands includes at least 6th power term of the power series.
(2-25) In the optical pickup apparatus described in (2-23) or (2-24), at least one surface of the objective lens is an aspherical surface.
(2-26) In the optical pickup apparatus described in (2-23) or (2-24), both surfaces of the objective lens represent an aspherical surface.
(2-27) An objective lens for use in an optical pickup apparatus comprises at least one plastic lens having refracting power and an axial spherical aberration change amount caused by ambient temperatures is corrected by making at least one surface to be an aspherical surface and spherical aberration is corrected by providing a diffraction pattern on at least one surface.
(2-28) In the objective lens for use in an optical pickup apparatus described in (2-27), the objective lens is composed of a single lens of the plastic lens.
(2-29) An objective lens for use in an optical pickup apparatus comprises a plastic lens having refracting power, the following expression is satisfied by third order axial spherical aberration change amount xcex94SAX when a light flux having wavelength xcex nm is made to enter the objective lens having a diffraction pattern on at least one surface thereof from a light source for measurement, and when the third order axial spherical aberration change amount xcex94SAX of the objective lens for temperature change xcex94Txc2x0 C. within a range of 20xc2x0 C.-30xc2x0 C. of ambient temperatures is measured,
xe2x80x83|xcex94SAX/xcex94T|xe2x89xa60.001 xcexrms/xc2x0 C.
(2-30) In the objective lens for use in an optical pickup apparatus described in (2-29), the measurement is conducted by an interferometer of a Fizeau type or by an interferometer of a Twyman-Green type.
(2-31) In the objective lens for use in an optical pickup apparatus described in (2-29) or (2-30), wavelength xcex nm of the light source for measurement stated above is 680 nm or less.
(2-32) In the objective lens for use in an optical pickup apparatus described in (2-29) or (2-30), the wavelength xcex nm of the light source for measurement stated above is 633 nm.
(2-33) In the objective lens for use in an optical pickup apparatus described in (2-29) or (2-30), the wavelength xcex nm of the light source for measurement stated above is not longer than 500 nm.
(2-34) In the objective lens for use in an optical pickup apparatus described in one of (2-29) to (2-33), a numerical aperture thereof on the side where a light flux is emitted is 0.58 or more.
(2-35) In the objective lens for use in an optical pickup apparatus described in one of (2-29) to (2-33), a numerical aperture thereof on the side where a light flux is emitted is 0.62 or more.
(2-36) In the objective lens for use in an optical pickup apparatus described in one of (2-29) to (2-35), an absolute value of the third order axial spherical aberration component of a wavefront aberration obtained through the measurement stated above is 0.07 xcexrms or less on ambient temperature range of 20xc2x0 C. to 30xc2x0 C.
(2-37) An objective lens for use in an optical pickup apparatus to conduct recording and/or reproduction of information for an optical information recording medium equipped with a transparent base board by using light from a light source, comprises a plastic lens having refracting power, and when a light source for measurement capable of emitting measurement light having the wavelength that is mostly the same as a light flux coming from a light source of the optical pickup apparatus to the objective lens and a measurement medium having the refractive index and thickness which are mostly the same as those of the transparent base board of the optical information recording medium are provided to be equal in terms of the relational position between the light source of the optical pickup apparatus and the transparent base board, and when the measurement light having wavelength xcex nm is made to enter the objective lens, and third order axial spherical aberration change amount xcex94SAX of the objective lens for temperature change xcex94T(xc2x0 C.) within a range of 20xc2x0 C.-30xc2x0 C. of the objective lens temperature is measured, the third order axial spherical aberration change amount xcex94SAX satisfies the following expression,
|xcex94SAX/xcex94T|xe2x89xa60.001 xcexrms/xc2x0 C.
(3-1) An optical pickup apparatus comprises therein a light source emitting a light flux having wavelength xcex1 nm, a light-converging optical system including an objective lens that converges a light flux emitted from the light source on an information recording surface of the optical information recording medium through a transparent base board of the optical information recording medium, and a photo-detector that receives reflected light from the optical information recording medium, and is characterized in that the objective lens is a plastic lens, and the following expressions are satisfied,
NA(1)xe2x89xa70.49 xe2x88x92xc2xdxe2x89xa6mo1xe2x89xa6xe2x88x92{fraction (1/7.5)}
when NA (1) represents a numerical aperture of the objective lens on the optical information recording medium side and mo1 represents an image forming magnification of the objective lens, and the following expression is satisfied when xcex94SA1 represents an amount of change in axial spherical aberration of the light-converging optical system for temperature change xcex94T (xc2x0 C.) in a range of 20xc2x0 C.-30xc2x0 C. of ambient temperatures.
|xcex94SA1/xcex94T|xe2x89xa60.0005 xcexrms/xc2x0 C.
(3-2) In the optical pickup apparatus described in (3-1), the following expression is satisfied when xcex94xcex1 nm represents an amount of change in wavelength of the light source for temperature change xcex94T (xc2x0 C.) in a range of 20xc2x0 C.-30xc2x0 C. of ambient temperatures,
0xe2x89xa6xcex94xcex1/xcex94Txe2x89xa60.5 nm/xc2x0 C.
(3-3) In the optical pickup apparatus described in (3-1), the following expression is satisfied when xcex94n1 represents an amount of change in refractive index of a material of the plastic lens for temperature change xcex94T (xc2x0 C.) in a range of 20xc2x0 C.-30xc2x0 C. of ambient temperatures in the case of wavelength xcex1 nm,
xe2x88x920.0002/xc2x0 C.xe2x89xa6xcex94n1/xcex94T less than xe2x88x920.00005/xc2x0 C.
(3-4) In the optical pickup apparatus described in one of (3-1) to (3-3), the objective lens is driven in the direction perpendicular to an optical axis of the objective lens for tracking and its relative position to the light source is changed, and thereby, the position where the astigmatism component of wavefront aberration of the light flux which has emerged from the objective lens is minimum is a position where the center of the light flux emitted from the light source is deviated from an optical axis of the objective lens.
(3-5) In the optical pickup apparatus described in one of (3-1) to (3-4), the following expression is satisfied when U represents a distance between the light source and the information recording surface of the optical information recording medium,
10 mm less than U less than 40 mm
(3-6) An optical pickup apparatus comprises a first light source with wavelength xcex1 nm, a second light source with wavelength xcex2 nm (xcex2 greater than xcex1 nm), a light-converging optical system including an objective lens that converges light fluxes emitted from the first light source and the second light source on an information recording surface of the optical information recording medium through a transparent base board of the optical information recording medium, and a photo-detector that receives reflected light representing a light flux emitted from each of the first and second light sources and is reflected on the optical information recording medium, recording and/or reproducing of information is conducted by the first light flux coming from the first light source for the first optical information recording medium wherein a thickness of a transparent base board is t1, recording and/or reproducing of information is conducted by the second light flux coming from the second light source for the second optical information recording medium wherein a thickness of a transparent base board is t2, the objective lens is a plastic lens, at least one surface of the objective lens has a diffraction pattern, the following expressions hold when NA1 represents a necessary numerical aperture of the light-converging optical system on the optical information recording medium side necessary for recording or reproducing the first optical information recording medium with wavelength xcex1, and when NA2 represents a necessary numerical aperture of the light-converging optical system on the optical information recording medium side necessary for recording or reproducing the second optical information recording medium with wavelength xcex2,
t1 less than t2
NA1 greater than NA2
the following expressions are satisfied when NA (1) represents a numerical aperture of the objective lens on the side of the optical information recording medium for the first light flux, and mo1 represents image forming magnification of the objective lens for the first light flux,
NA(1)xe2x89xa70.56xe2x88x92⅕xe2x89xa6mo1xe2x89xa6xe2x88x92{fraction (1/7.5)}
and the following expression is satisfied when xcex94SA1 represents an amount of change in axial spherical aberration of the light-converging optical system for temperature change xcex94T (xc2x0 C.) in a range of 20xc2x0 C.-30xc2x0 C. of ambient temperatures,
|xcex94SA1/xcex94T|xe2x89xa60.0005 xcexrms/xc2x0 C.
(3-7) In the optical pickup apparatus described in (3-6), the following expression holds when mo2 represents image forming magnification of the objective lens for the second light flux:
|mo2xe2x88x92mo1|xe2x89xa60.10
(3-8) In the optical pickup apparatus described in (3-6) or (3-7), the optical pickup apparatus comprises a light mixing means capable of mixing the first light flux and the second light flux.
(3-9) In the optical pickup apparatus described in one of (3-6) to (3-7), an aperture regulating means which allows the first light flux and the central portion of the second light flux to pass through and prohibits the outer area of the second light flux to pass through is provided on the optical path through which the first and second light fluxes pass. (3-10) In the optical pickup apparatus described in (3-9), the aperture regulating means stated above is integrated solidly with the objective lens.
(3-11) In the optical pickup apparatus described in (3-10), the aperture regulating means integrated solidly with the objective lens is provided on the surface on one side of the objective lens and is of the partial dichroic coating which allows the first light flux and the central portion of the second light flux to pass through and reflects the outer area of the second light flux. (3-12) In the optical pickup apparatus described in (3-11), the diffraction pattern is provided only on the surface on one side of the objective lens and the partial dichroic coating is provided on the surface where no diffraction pattern is provided.
(3-13) In the optical pickup apparatus described in (3-11) or (3-12), the reflectance of the partial dichroic coating for a light flux having wavelength xcex2 is in a range from 30% to 70%.
(3-14) In the optical pickup apparatus described in (3-10), each of both surfaces of the objective lens has a diffraction pattern, and the aperture regulating means integrated solidly with the objective lens is a partial diffraction pattern that is provided on the surface on one side of the objective lens to allow the first light flux and the central portion of the second light flux to pass through and to diffract the outer area of the second light flux.
(3-15) In the optical pickup apparatus described in one of (3-6) to (3-14), a beam spot is formed by utilizing mainly an inner light flux and an outer light flux in a light flux emitted from the first light source, and information is recorded and/or reproduced for the first optical information recording medium, and a beam spot is formed by utilizing mainly an inner light flux and an intermediate light flux in a light flux emitted from the second light source, and information is recorded and/or reproduced for the second optical information recording medium, when a light flux entering the information recording surface is divided into at least three light fluxes including an inner light flux near an optical axis, an intermediate light flux that is located outside the inner light flux, and an outer light flux that is located outside the intermediate light flux.
(3-16) In the optical pickup apparatus described in (3-15), third order spherical aberration component of wavefront aberration of the inner area that enters an information recording surface of the second optical information recording medium in a light flux emitted from the second light source is under.
(3-17) In the optical pickup apparatus described in (3-16), the photo-detector is common for both the first light source and the second light source.
(3-18) In the optical pickup apparatus described in one of (3-6) to (3-16), the first light source is equipped with the first photo-detector and the second light source is equipped with the second photo-detector separately, and the first photo-detector and the second photo-detector are positioned to be away from each other spatially.
(3-19) In the optical pickup apparatus described in (3-18), at least the first light source and the first photo-detector, or the second light source and the second photo-detector are integrally unitized.
(3-20) In the optical pickup apparatus described in (3-17), the first light source, the second light source and the common photo-detector (a single photo-detector) are integrally unitized.
(3-21) In the optical pickup apparatus described in one of (3-6) to (3-16), the first photo-detector for the first light source is separate from the second photo-detector for the second light source with regard to the photo-detector, and the first light source, the second light source, the first photo-detector and the second photo-detector are integrally unitized.
(3-22) In the optical pickup apparatus described in one of (3-6) to (3-16), the first light source and the second light source are integrally unitized and are positioned to be away from the photo-detector spatially.
(3-23) In the optical pickup apparatus described in one of (3-6) to (3-22), a coupling lens that makes divergence of a light flux emitted from a light source to be small is provided in at least one of an optical path from the first light source to the objective lens and an optical path from the second light source to the objective lens.
(3-24) An optical pickup apparatus comprises a light source with wavelength xcex1 nm, a light-converging optical system including an objective lens that converges a light flux emitted from the light source-on an information recording surface of the optical information recording medium through a transparent base board of the optical information recording medium, and a photo-detector that receives reflected light representing a light flux emitted from the light source and is reflected on the optical information recording medium, recording and/or reproducing of information is conducted by the light flux coming from the light source for the first optical information recording medium wherein a thickness of a transparent base board is t1 and for the second optical information recording medium wherein a thickness of a transparent base board is t2, the objective lens is a plastic lens, at least one surface of the objective lens has a diffraction pattern, the following expressions hold when NA1 represents a necessary numerical aperture of the light-converging optical system on the optical information recording medium side necessary for recording or reproducing the first optical information recording medium with wavelength xcex1, and when NA2 represents a necessary numerical aperture of the light-converging optical system on the optical information recording medium side necessary for recording or reproducing the second optical information recording medium with wavelength xcex1,
t1 less than t2
NA1 greater than NA2
the following expression is satisfied when mo1 represents image forming magnification of the objective lens,
xe2x88x92⅕xe2x89xa6mo1xe2x89xa6xe2x88x92{fraction (1/7.5)}
the following expression is satisfied when xcex94SA1 represents an amount of change in axial spherical aberration of the light-converging optical system for temperature change xcex94T (xc2x0 C.) in a range of 20xc2x0 C.-30xc2x0 C. of ambient temperatures,
xe2x80x83|xcex94SA1/xcex94T|xe2x89xa60.0005 xcexrms/xc2x0 C.
and there is provided an aperture regulating means which allows a central portion of the light flux emitted from a light source to pass through and intercepts an outer area of the light flux when conducting recording and/or reproducing for the second optical information recording medium.
(3-25) An objective lens is an objective lens for use in an optical pickup apparatus having therein a light source emitting a light flux with wavelength xcex1 nm, a light-converging optical system including an objective lens that converges a light flux emitted from the light source on an information recording surface of the optical information recording medium through a transparent base board of the optical information recording medium, and a photo-detector that receives reflected light coming from the optical information recording medium, and the objective lens is a plastic lens, at least one surface of the objective lens has a diffraction pattern, the following expressions hold when NA (1) represents a numerical aperture of the objective lens on the optical information recording medium side,
NA(1)xe2x89xa70.49xe2x88x92xc2xdxe2x89xa6mo1xe2x89xa6xe2x88x92{fraction (1/7.5)}
and the following expression is satisfied when xcex94SA1represents an amount of change in axial spherical aberration of the light-converging optical system for temperature change xcex94T (xc2x0 C.) in a range of 20xc2x0 C.-30xc2x0 C. of ambient temperatures:
xcex94SA1/xcex94T|xe2x89xa60.0005 xcexrms/xc2x0 C.
(3-26) In the objective lens described in (3-25), the following expression is satisfied when xcex94xcex1 represents an amount of change in wavelength of the light source for temperature change xcex94T (xc2x0 C.) in a range of 20xc2x0 C.-30xc2x0 C. of ambient temperatures:
0xe2x89xa6xcex1xcex1/xcex94Txe2x89xa60.5 nm/xc2x0 C.
(3-27) In the objective lens described in (3-25) or (3-26), the following expression is satisfied when xcex94n1 represents an amount of change in refractive index of materials of the plastic lens for temperature change xcex94T (xc2x0 C.) in a range of 20xc2x0 C.-30xc2x0 C. of ambient temperatures:
xe2x88x920.0002/xc2x0 C. less than xcex94n1/xcex94Txe2x89xa6xe2x88x920.00005/xc2x0 C.
(3-28) In the objective lens described in one of (3-25) to (3-27), the relative position between the objective lens and the light source is changed when the objective lens is driven in the direction perpendicular to an optical axis of the objective lens for the reason of tracking in the optical pickup apparatus, and the position where the astigmatism component of wavefront aberration of the light flux emerged out of the objective lens is minimum is a position where the center of a light flux emitted from the light source is deviated from an optical axis of the objective lens.
(3-29) In the objective lens described in one of (3-25) to (3-28), the following expression is satisfied when U represents a distance between the light source and an information recording surface of the optical information recording medium in the optical pickup apparatus:
10 mmxe2x89xa6Uxe2x89xa640 mm
(3-30) An optical pickup apparatus is an objective lens of an optical pickup apparatus having therein a first light source with wavelength xcex1 nm, a second light source with wavelength xcex2 nm (xcex2 greater than xcex1), a light-converging optical system including an objective lens that converges a light flux emitted from the first light source and a light source emitted from the second light source on an information recording surface of the optical information recording medium through a transparent base board of the optical information recording medium, and a photo-detector that receives reflected light representing a light flux emitted from the first light source and a light flux emitted from the second light source reflected on the optical information recording medium, wherein recording and/or reproducing of information is conducted by the first light flux emitted from the first light source for the first optical information recording medium wherein a thickness of the transparent base board is t1, and recording and/or reproducing of information is conducted by the second light flux emitted from the second light source for the second optical information recording medium wherein a thickness of the transparent base board is t2, the objective lens is a plastic lens, at least one surface of the objective lens has a diffraction pattern, the following expressions hold when NA1 represents a necessary numerical aperture of the light-converging optical system on the optical information recording medium side necessary for recording or reproducing the first optical information recording medium with wavelength xcex1, and when NA2 represents a necessary numerical aperture of the light-converging optical system on the optical information recording medium side necessary for recording or reproducing the second optical information recording medium with wavelength xcex2,
t1 less than t2
NA1 greater than NA2
the following expressions are satisfied when NA (1) represents a numerical aperture of the objective lens on the optical information recording medium side for the first light flux and mo1 represents image forming magnification of the objective lens for the first light flux,
NA(1)xe2x89xa70.56xe2x88x92⅕xe2x89xa6mo1xe2x89xa6xe2x88x92{fraction (1/7.5)}
and the following expression is satisfied when xcex94SA1 represents an amount of change in axial spherical aberration of the light-converging optical system for temperature change xcex94T (xc2x0 C.) in a range of 20xc2x0 C.-30xc2x0 C. of ambient temperatures:
|xcex94SA1/xcex94T|xe2x89xa60.0005 xcexrms/xc2x0 C.
(3-31) In the objective lens described in (3-30), the following expression is satisfied when mo2 represents an image forming magnification of the objective lens for the second light flux:
|mo2xe2x88x92mo1| less than 0.10
(3-32) In the objective lens described in (3-30) or (3-31), the optical pickup apparatus has light mixing means capable of mixing the first light flux and the second light flux.
(3-33) In the objective lens described in one of (3-30) to (3-32), an aperture regulating means which allows the first light flux and a central portion of the second light flux to pass through and intercepts an outer area of the second light flux is provided in the optical path through which the first light flux and the second light flux pass commonly, in the optical pickup apparatus.
(3-34) In the objective lens described in (3-33), the aperture regulating means is unitized integrally with the objective lens.
(3-35) In the objective lens described in (3-34), the aperture regulating means unitized integrally with the objective lens is provided on the surface on one side of the objective lens, and it represents partial dichroic coating that allows the first light flux and a central portion of the second light flux to pass through it and reflects an outer area of the second light flux.
(3-36) In the objective lens described in (3-35), the diffraction pattern is provided on only one side of the objective lens, and the partial dichroic coating is provided on the side where no diffraction pattern is provided. (3-37) In the objective lens described in (3-35) or (3-36), the reflectance of the partial dichroic coating for the light flux with wavelength xcex2 is within a range of 30%-70%.
(3-38) In the objective lens described in (3-34), diffraction patterns are provided on both sides of the objective lens, and the aperture regulating means unitized with the objective lens represents partial diffraction pattern which allows the first light flux provided on the surface on one side of the objective lens and a central portion of the second light flux to pass through and diffracts an outer area of the second light flux.
(3-39) In the objective lens described in one of (3-30) to (3-34), a beam spot is formed by using mainly an inner light flux and an outer light flux in a light flux emitted from the first light source, and recording and/or reproducing of information is conducted for the first optical information recording medium, and a beam spot is formed by using mainly an inner light flux and an intermediate light flux in a light flux emitted from the second light source, and recording and/or reproducing of information is conducted for the second optical information recording medium, when a light flux entering the information recording surface is divided into an inner light flux near an optical axis, an intermediate light flux that is outside the inner light flux, and an outer light flux that is outside the intermediate light flux, in the optical pickup apparatus.
(3-40) In the objective lens described in (3-39), third order spherical aberration component of wavefront aberration of the inner area that enters an information recording surface of the second optical information recording medium in a light flux emitted from the second light source is under, in the optical pickup apparatus.
(3-41) In the objective lens described in one of (3-30) to (3-40), the photo-detector of the optical pickup apparatus is common for both the first light source and the second light source.
(3-42) In the objective lens described in one of (3-30) to (3-40), the first light source is equipped with the first photo-detector and the second light source is equipped with the second photo-detector separately, and the first photo-detector and the second photo-detector are positioned to be away from each other spatially.
(3-43) In the objective lens described in (3-42), at least a pair of the first light source and the first photo-detector or a pair of the second light source and the second photo-detector is unitized, in the optical pickup apparatus.
(3-44) In the objective lens described in (3-41), the first light source, the second light source and the common photo-detector (a single photo-detector) are unitized in the optical pickup apparatus.
(3-45) In the objective lens described in one of (3-30) to (3-40), the first photo-detector for the first light source and the second photo-detector for the second light source are separate each other in the photo-detector of the optical pickup apparatus, and the first light source, the second light source, the first photo-detector and the second photo-detector are unitized.
(3-46) In the objective lens described in one of (3-30) to (3-40), the first light source and the second light source are unitized, and they are located to be away from the photo-detector spatially, in the optical pickup apparatus.
(3-47) In the objective lens described in one of (3-30) to (3-46), a coupling lens that makes divergence of a light flux emitted from a light source to be small is provided in at least one of an optical path from the first light source to the objective lens and an optical path from the second light source to the objective lens, in the optical pickup apparatus.
(3-48) An objective lens is an objective lens for an optical pickup apparatus having therein a light source emitting a light flux with wavelength xcex1 nm, a light-converging optical system including an objective lens that converges a light flux emitted from the light source on an information recording surface of the optical information recording medium through a transparent base board of the optical information recording medium, and a photo-detector that receives reflected light coming from the optical information recording medium, wherein the optical pickup apparatus conducts recording or reproducing of information using a light flux emitted from the light source, for the first optical information recording medium whose transparent base board has a thickness of t1 and for the second optical information recording medium whose transparent base board has a thickness of t2, the objective lens is a plastic lens, at least one surface of the objective lens has a diffraction pattern, the following expressions hold when NA1 represents a necessary numerical aperture of the light-converging optical system on the optical information recording medium side necessary for recording or reproducing the first optical information recording medium with wavelength xcex1, and when NA2 represents a necessary numerical aperture of the light-converging optical system on the optical information recording medium side necessary for recording or reproducing the second optical information recording medium with wavelength xcex1,
t1xe2x89xa6t2
NA1 greater than NA2
the following expression is satisfied when mo1 represents image forming magnification of the objective lens,
xe2x88x92⅕xe2x89xa6mo1xe2x89xa6xe2x88x92{fraction (1/7.5)}
the following expression is satisfied when xcex94SA1 represents an amount of change in axial spherical aberration of the light-converging optical system for temperature change xcex94T (xc2x0 C.) in a range of 20xc2x0 C.-30xc2x0 C. of ambient temperatures,
|xcex94SA1/xcex94T|xe2x89xa60.0005 xcexrms/xc2x0 C.
and there is provided an aperture regulating means that transmits the central portion of a light flux emitted from a light source and intercepts an outer area thereof for recording and/or reproducing of the second optical information recording medium.
(3-49) An objective lens for use in an optical pickup apparatus is a plastic lens having a diffraction pattern on at least one surface thereof which is characterized in that the following expression holds when xcex94Z represents an amount of astigmatism:
0.2 xcexcm less than xcex94Z less than 0.7 xcexcm
(3-50) In the objective lens for use in an optical pickup apparatus described in (3-49), an axial chromatic aberration is over-corrected in the vicinity of the wavelength used.
(3-51) An objective lens is a plastic lens and the following expression is satisfied when Mmin represents image forming magnification of the objective lens at which the third order spherical aberration component of the wavefront aberration is minimum when a light source with wavelength of 620 nm-680 nm is arranged and a polycarbonate transparent base board having a thickness of 0.6 mm is arranged on the side opposite to that for the light source and the third order spherical aberration component is measured through the transparent base board,
xe2x88x92⅕xe2x89xa6Mminxe2x89xa6xe2x88x92{fraction (1/12)}
and at least one surface has thereon a diffraction pattern.
(3-52) In the objective lens for use in an optical pickup apparatus described in (3-51), the following expression is satisfied by the image forming magnification Mmin.
xe2x88x92⅕xe2x89xa6Mminxe2x89xa6xe2x88x92{fraction (1/7.5)}